This section is intended to introduce the reader to various aspects of the art that may be related to various aspects of the present invention. The following discussion is intended to provide information to facilitate a better understanding of the present invention. Accordingly, it should be understood that statements in the following discussion are to be read in this light, and not as admissions of prior art.
The present invention describes an approach to eliminate or suppress signals from physical regions such that the magnetic resonance images formed by the process will experience regions of signal loss applied in a controlled manner. Selective elimination of signal is beneficial in several areas, including allowing inner volume imaging in 2D and 3D data sets, suppressing signal along an arbitrary line intersecting the imaged plane to allow direct visualization of flowing blood, and to efficiently obtain black-blood cine images. The technique is termed Elimination of Regionally Acquired Signal using Echo Refocusing: ERASER. The underlying imaging approach used is steady-state-free-precession (SSFP) magnetic resonance imaging (MRI). In SSFP imaging, a high steady state signal is established and maintained by applying radio frequency (RF) pulses in a repetitive manner, over a time scale much shorter than the relaxation time of the spin system. For imaging sequences using SSFP the imaging gradients are applied in a manner such that their area balances to zero over each TR period. Any residual gradients, either applied as part of the imaging sequence, or gradients that are present due to inhomogeneities of the main magnetic field, will dephase the spin system in a spatially dependant manner, and conventionally, great care is taken to eliminate such residual field gradients. When the spins are dephased by close to 90 degrees from their ideal phase of zero degrees, the spins will not refocus when the RF pulses are repetitively applied at the TR interval. Typically, these spins are present as artifacts in conventional images, and typically the region of signal loss is limited to a relatively thin line or set of lines (FIG. 1). FIG. 1 shows that regions of signal loss occur as a consequence of main magnetic field inhomogeneities, and typically manifest as curved lines of low or zero signal. Typically, these low signal regions are confined to the outer regions of the body. However, different inhomogeneity patterns can produce wider bands of signal loss (FIG. 2). FIG. 2 shows that when deliberately introducing a focal magnetic field inhomogeneity (in this case by introducing a static steel paper clip into the imaging field) a strong pattern of alternating dark and bright signal regions can be seen. In this case the inhomogeneity gradient is non-linear, leading to non-linear spacing of the bright and dark bands, but where the gradient approximates to a linear region, the width of the bright and dark bands are approximately equal. In ERASER, conditions are arranged to bring this phenomena under control, and to enhance its effectiveness to preferentially diminish or eliminate signals from targeted spatial regions in the image.
The closest known technology is missing pulse SSFP (MP-SSFP) imaging. In the MP-SSFP approach, intersecting spatially selective slices and slabs are proposed. However, signal is not acquired immediately following application of each RF pulse, rather, signal is acquired during the period when a subsequent “missing pulse” would normally be applied, i.e. if two intersecting spatial regions were applied, the signal originating only from the intersecting region would be acquired at the time of the (missing) third RF pulse. Thus, at a minimum, signal is only acquired every third TR period, and at longer intervals if a higher number of intersecting pulses are used. Further. MP-SSFP does not require application of a spin-dephasing gradient.